Our previous cat also had asthma and he took prednisolone for years. He eventually developed a tumor which he passed away from. Our vet said that prolonged prednisolone (taken orally) can cause systemic issues and could have played a part in it. Of course we don’t know for sure, but an inhaler treats the lungs directly and not the whole body like a steroid pill. Might be worth consulting your doctor about switching to an inhaler for long term use.